1. Field of the Invention
The present invention relates to an improved process for carrying out hydrogenations, in particular hydrodesulfurization in a catalyst bed.
2. Related Art
The most common method of removal of the sulfur compounds is by hydrodesulfurization (HDS) in which the petroleum feed is passed over a solid particulate catalyst comprising a hydrogenation metal supported on an alumina base. Additionally copious quantities of hydrogen are included in the feed. The following equations illustrate the reactions in a typical HDS unit:
(1) RSH+H2xe2x86x92RH+H2S
(2) RCl+H2xe2x86x92RH+HCl
(3) 2RN+4H2xe2x86x92RH+NH3 
(4) ROOH+2H2xe2x86x92RH+H2O
Typical operating conditions for the HDS reactions are:
After the hydrotreating is complete, the product may be fractionated or simply flashed to release the hydrogen sulfide and collect the now desulfurized material. Olefinically unsaturated compounds may also be hydrogenated. The order of decreasing activity is:
diolefins
mono olefins
Trickle bed reactors have been used in this service for more than thirty years. Generally the trickle bed reactors use a fixed catalyst bed having a hydrogenation metal catalyst in one or more layers through which the stream to be hydrogenated is passed with excess hydrogen. Most reactors are downflow with hydrogen either concurrentflow or counterflow to the petroleum feed stream. Depending on the process the petroleum feed to the reactor may be vaporous, liquid or mixed phase and the products may be vaporous, liquid or mixed phase. In all of these processes the commonality has been high pressure, i.e., in excess of 300 psig up to 3000 psig and long residence times.
The present invention maintains a liquid phase in the reaction zone and also provides a means for removing heat from the fixed continuous catalyst bed. A substantial portion of the sulfur is converted to H2S by hydrodesulfurization and is easily distilled away from the hydrocarbons. It is a further advantage that the present type of reaction may be used in conjunction with a catalytic distillation column reactor to obtain a very high degree of sulfur removal from the feed stream. These and other advantages will become apparent from the following descriptions.
The present invention is a process of hydrotreating petroleum feed comprising concurrently passing a petroleum feed containing organic sulfur compounds and hydrogen downflow through a reaction zone containing a hydrodesulfurization catalyst at a pressure of less than 300 psig pressure, preferably less than 275 psig, for example less than 200 psig, and for example at least about 100 psig at a temperature within the range of 300xc2x0 F. to 700xc2x0 F. to produce an effluent, said temperature and pressure being adjusted such that the temperature of the effluent is above its boiling point and below its dew point, whereby at least a portion but less than all of the material in said reaction zone is in the vapor phase and a portion of the organic sulfur compounds are converted to H2S. Preferably the weight hourly space velocity (WHSV), i.e., the weight of petroleum feed per hour per volume of catalyst is greater than 6 hrxe2x88x921, preferably greater than 8 hrxe2x88x921 and more preferably greater than 15 hrxe2x88x921.
The reaction mixture (which includes the petroleum feed and the hydrotreated petroleum products), will have different boiling points at different pressures, hence the temperature in the reactor may be controlled by adjusting the pressure to the desired temperature within the recited range. The boiling point of the reaction mixture thus is the temperature of the reaction and the exothermic heat of reaction is dissipated by vaporization of the reaction mixture. The maximum temperature of any heated liquid composition will be the boiling point of the composition at a given pressure with additional heat merely causing more boilup. There must be liquid present, however, to provide the boil up, otherwise the temperature in the reactor will continue to rise which may damage the catalyst or cause coking. The temperature in the reaction zone is preferably not higher than the dew point of the reaction effluent, thus guaranteeing the presence of the liquid in the reaction. The feed to the reaction is preferably at least partially liquid phase.
To fully appreciate this aspect of the present invention, one must recognize that the petroleum feed, the reaction mixture and the reaction effluent form a very complex mixture of hydrocarbons, boiling over a range of temperatures and that similarly there is a range of dew points. Thus, the actual temperature of the reaction effluent (which is very similar in composition to that of the petroleum feed but having a reduced olefin content which also occurs during the sulfur compound removal) is the temperature at a given pressure at which some lower boiling components are vaporized, but at which some of the higher boiling components are not boiling, i.e., some higher boiling components are below their dew point. Therefore, in the present reaction system there are always two phases. It is believed that the presence of the liquid phase as described herein allows the lower pressures and shorter residence times (high space velocities).
The nature of some streams that are treated according to the present process is such that within the process operating variables, the steam is totally vaporized and thus the benefit of the invention is not obtained. In these cases a higher boiling petroleum component is added to the stream, i.e., the xe2x80x9ctargetxe2x80x9d stream to be treated and the conditions adjusted so as to vaporize whatever portion of the target stream is necessary to reduce the total sulfur content, while the higher boiling petroleum component provides the liquid component of the reaction system.
In a preferred embodiment the catalyst bed may be described as a fixed continuous bed, that is, the catalyst is loaded into the reactor in its particulate form to fill the reactor or reaction zone, although there may be one or more such continuous beds in a reactor, separated by spaces devoid of catalyst.
As used herein the term xe2x80x9cdistillation column reactorxe2x80x9d means a distillation column which also contains catalysts such that reaction and distillation are going on concurrently in the column. In a preferred embodiment the catalyst is prepared as a distillation structure and serves as both the catalyst and distillation structure.